This invention relates to a process for extruding a liquid or paste viscosity hydrolyzable siloxane composition through a shaping die and curing the shaped extrusion into a cured elastomeric article without distorting the shape.
Siloxane elastomer extrusions have typically been formed from a curable siloxane composition having a firm, self-supporting putty consistency. The curable composition is extruded through a shaping die to form an extrusion having a desired shape. Because the curable siloxane composition has a firm, self-supporting putty consistency, high pressure is required to force the composition through the extruder and shaping die. This makes necessary the use of a rugged extruder and shaping die. Because of the high pressure forcing the composition through the die, the extrusion normally swells upon leaving the die, which is known as die swell. Die swell leads to problems in obtaining exact and consistent dimensions in the cured, shaped extrusion. The high pressure also causes a heat buildup in the extruder which has to be countered during continuous operation with cooling channels in the extruder. After leaving the extruder, the shaped extrusion is heat cured through exposure to steam in a continuous steam vulcanizer or through exposure to hot air in a continuous hot air vulcanizer. The shaped extrusion is supported on a solid surface during heat curing.
Hot liquid vulcanization has been described as a method for curing non-self-supporting silicone rubber extrusions in the The Handbook of Silicone Rubber Fabrication. Wilfred Lynch, Van Nostrand Reinhold Company, New York, N.Y., 1978. Shimizu et al., in U.S. Pat. No. 4,783,289, issued Nov. 8, 1988, disclose a process for fabricating an extruded shaped article from a curable liquid silicone rubber composition which is self-supporting or non-self-supporting and which cures by addition-reaction, free-radical initiation or condensation. The disclosed process includes mixing the ingredients of the composition at a temperature of from -60.degree. C. to 5.degree. C. and extruding the mixed composition into a water bath having a temperature of at least 25.degree. C. in which the mixed composition is submerged. The patent discloses a process employing compositions comprising an alkenyl-containing polydiorganosiloxane and either an organoperoxide or a combination of a polydiorganosiloxane having at least two silicon-bonded hydrogen atoms in each molecule and a platinum-group catalyst. The disclosed compositions include only those which cure by an addition-reaction mechanism or by a free-radical initiation mechanism.
The present process employs a non-self-supporting, hydrolyzable siloxane composition which cures through a condensation reaction upon exposure to moisture. The composition comprises a polydiorganosiloxane having at least two silicon-bonded hydrolyzable groups per molecule, a hydrolyzable silicon crosslinker, a filler, and optionally a curing catalyst, which are mixed together at a temperature greater than 5.degree. C., preferably greater than 10.degree. C. The process employs a constant extrusion rate which, in combination with the flowability of the composition, allows the composition to be extruded through a shaping die with a much lesser degree of die swell than is encountered with compositions having a self-supporting putty viscosity. This allows production of a shaped extrusion which is substantially uniform when leaving the die and being fed into an aqueous curing bath. In contrast to the addition-reaction and free-radical curing mechanisms disclosed in the Shimizu patent, a condensation curing mechanism can cause shrinkage of the shaped extrusion upon cure as the hydrolyzable groups come off of the polydiorganosiloxane, crosslinker, and filler. This shrinkage can result in non-uniformity of the cured article produced from the shaped extrusion. The process of the present invention allows preparation of a cured article having a substantially uniform cross-sectional area throughout its length despite the shrinkage that inherently results from curing through a condensation reaction.